The present invention relates to a method for liquefaction of gas, particularly natural gas, using multicomponent refrigerant.
Liquefaction of gas, particularly natural gas, is well known from larger industrial plants, so called xe2x80x9cbaseloadxe2x80x9d plants, and from peak shaving plants. Such plants have the property in common that they convert a substantial quantum gas pr time, so they can bear a significant upfront investment. The costs pr gas volume will still be relatively low over time. Multicomponent refrigerants are commonly used for such plants, as this is the most effective way to reach the sufficiently low temperatures.
Kleemenko (10th International Congress of Refrigeration, 1959) describes a process for multicomponent cooling and liquefaction of natural gas, based on use of multiflow heat exchangers.
U.S. Pat. No. 3,593,535 describes a plant for the same purpose, based on three-flow spiral heat exchangers with a an upward flow direction for the condensing fluid and a downward flow direction for the vaporizing fluid.
A similar plant is known from U.S. Pat. No. 3,364,685, in which however the heat exchangers are two-flow heat exchangers over two steps of pressure and with flow directions as mentioned above.
U.S. Pat. No. 2,041,745 describes a plant for liquefaction of natural gas partly based on two-flow heat exchangers, where the most volatile component of the refrigerant is condensed out in an open process. In such an open process it is required that the gas composition is adapted to the purpose. Closed processes are generally more versatile.
There is however, a need for liquefaction of gas, particularly natural gas, many places where it is not possible to enjoy large scale benefits, for instance in connection with local distribution of natural gas, where the plant is to be arranged at a gas pipe, while the liquefied gas is transported by trucks, small ships or the like. For such situations there is a need for smaller and less expensive plants.
Small plants will also be convenient in connection with small gas fields, for example of so called associated gas, or in connection with larger plants where it is desired to avoid flaring of the gas. In the following the term xe2x80x9cproduct gasxe2x80x9d is used synonymously with natural gas.
For such plants it is more important with low investment costs than optimal energy optimization. Furthermore a small plant may be factory assembled and transported to the site of use in one or several standard containers.
It is thus an object of the present invention to provide a method and a process plant for the liquefaction of gas, particularly natural gas, that is adapted for small and medium sized scale liquefaction.
It is furthermore an object to provide a plant for the liquefaction of gas for which the investment costs are modest.
It is thus a derived object to provide a method and a small scale process plant for cooling and liquefaction of gas, particularly natural gas, with a multicomponent refrigerant, where the plant is solely based on conventional two-flow plate heat exchangers and conventional oil lubricated compressors. It is furthermore a derived object to provide a small scale plant for the liquefaction of natural gas, which plant may be transported factory assembled to the site of use.
With the plant according to the invention there is obtained a small scale plant for cooling and liquefaction, where the plant costs is not prohibitive of a cost-effective operation. By the way with which the components of the plant are combined, it is avoided that oil from the compressors, which to some extent will contaminate the refrigerant, follows the flow of refrigerant to the coldest parts of the plant. It is thus avoided that the oil freezes and plugs conduits etc., which is an essential part of the invention.
To obtain this it has been necessary to include equipment for distribution of refrigerant between pairs of heat exchangers in separate rows, where the heat exchangers that cool the product flow is denoted primary heat exchangers and the heat exchangers that cool/heat different components of the multicomponent refrigerant are denoted secondary heat exchangers. The primary and secondary heat exchangers may be of same type and have similar dimensions, but the number of plates will depend upon the flow rate through the heat exchangers.
Use of multicomponent refrigerant is known per se, while achieving the benefits inherent with being able to reach very low temperatures in a simple plant, based on conventional components, is not. With the plant according to the invention is also obtained a natural flow direction in the plant, namely so that evaporating fluid moves upward while condensing fluid moves downward, avoiding that gravity negatively interferes with the process.